In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Drilling tools have been developed that, contrary to solid drills, are composed of two parts, including a basic or drill body and a head detachably connected with the same and thereby being replaceable. The head includes the requisite cutting edges. In such a way, the major part of the tool can be manufactured from a comparatively inexpensive material having a moderate modulus of elasticity, such as steel, while a smaller part, the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the cutting edges a good chip-removing capacity, a good machining precision and a long service life. The head forms a wear part that can be discarded after wear-out, while the basic body can be re-used several times, for example, 10 to 20 replacements. A now recognized term for these cutting edge-carrying heads is “loose tops”, which henceforth will be used in this document.
Loose top type drilling tools have a plurality of desired capabilities, one of which is that torque should be transferable in a reliable way from the rotatable, driven basic body to the loose top. Furthermore, the basic body should without problems be able to carry the rearwardly directed axial forces that the loose top is subjected to during drilling. Further, the loose top should be held centered in an exact and reliable way in relation to the basic body. Also, the loose top is clamped to the basic body not only during drilling of a hole, but also during retraction of the drilling tool out of the same. A user further desires that the loose top should be mountable and dismountable in a rapid and convenient way without the basic body necessarily having to be removed from the driving machine. In addition, the tool, and in particular the loose top manufactured from expensive materials, should be capable of low cost manufacture.
A loose-top tool intended for drilling and of the initially generally mentioned kind is previously known by EP 1013367. In this case, the two branches of the basic body are arranged to be turned into arched pockets, which are recessed in the rear part of convex envelope surfaces of two bars included in the loose top and separated by chip flutes, and which have a limited axial extension that in turn limits the maximally possible length of the branches. The internal support surfaces of the branches and the external side contact surfaces of the loose top, which are pressed against each other in order to resiliently and securely pinch the loose top in the jaw between the branches, have a rotationally symmetrical basic shape. The external side contact surfaces of the loose top generally have a larger diametrical dimension than the inner support surfaces of the branches in order to bend out the branches elastically or resiliently. In their angle-wise end position of turning, the rotationally heading, torque-transferring tangential support surfaces of the branches should be pressed into close contact against two tangential contact surfaces that form end surfaces in the two pockets in the loose top.
The tool of EP 1013367 is meritorious in several respects, one of which is that the axial support surface that is situated between the branches and forms a bottom in the jaw of the basic body does not need to be intersected by any slot or cavity in which chips could get caught. Another merit is that the loose top can be made fairly short in relation to its diameter, something that is material-saving and cost-reducing. In addition, the axial contact surface of the loose top as well as the axial support surface of the basic body extends between ends that are peripherally situated. In such a way, these surfaces become ample and thereby suitable to transfer great axial forces.
A disadvantage of the known tool is, however, that the mounting of the loose top in the jaw of the basic body risks becoming unreliable and cumbersome to carry out. Already when the two branches initially begin to be turned into the appurtenant pockets in the loose top, the branches are subjected to a clamping force that from then on becomes equally great during the entire rotary motion up to the end position in which the branches are pressed against the end surfaces of the pockets. Because the mounting is carried out in a manual way and the branches are held resiliently clamped against the side contact surfaces of the loose top by a force that is equally great during the entire rotary motion, it may become difficult for the operator to determine whether the loose top has reached its end position or not. This decision is made more difficult by the fact that the uniform clamping force has to be fairly great in order for the loose top to be clamped reasonably reliably. This means that the work with the turning-in becomes laborious, and therefore the operator, particularly when in a hurry, may unintentionally finish the turning-in too early, before the loose top has reached its end position in the jaw. Incorrect mounting of the loose top may, among other things, manifest itself in lost centering of the drilling tool in connection with the entering of a workpiece.